Dysregulated T cells are a major contributor to SLE pathogenesis, and their targeting is an important focus for therapeutic intervention. In the NZM2410 mouse model of lupus, we have determined that the Sle1 locus drives the production of anti-nuclear autoantibodies (ANA). Within Sle1, we have identified two loci, Sle1a1 and Sle1c2, which independently induce the production of activated chromatin-specific CD4+ T cells that provide help to ANA secreting B cells. Importantly, we have demonstrated that Sle1a1 and Sle1c2 each significantly enhance ANA production and immune activation either in in vivo models of lupus pathogenesis. During the last cycle of funding, we have functionally characterized these two loci and we have identified by genetic mapping and candidate gene analysis Pbx1 and Esrrg as the respective genes responsible for Sle1a1 and Sle1c2 phenotypes. The NZM2410 allele of Pbx1 is a novel splice isoform, Pbx1-d, which is expressed at a higher frequency in the CD4+ T cells from SLE patients than healthy controls (HC). Pbx1-d functions as a dominant negative and impairs CD4+ T cell responses to TGF and all-trans retinoic acid (ATRA), processes recently recognized to play a pivotal role in T cell effector functions. Esrrg regulates oxidative metabolism and its expression is significantly reduced in NZM2410 CD4+ T cells. Metabolic substrate utilization has been identified very recently as another defining mechanism of T cell functions. We hypothesize that Pbx1 and Esrrg contributes to lupus by each regulating CD4+ T cell effector functions through novel mechanisms. The goal of this proposal is to test this hypothesis in the mouse and assess our findings in lupus patients. To achieve this goal, we propose three specific aims, the first two in the mouse, and the third with lupus patients. Aim 1. To determine the mechanisms by Pbx1-d induce autoreactive CD4+ T cells. We will A) identify the genes differentially targeted by Pbx1 and Pbx1-d by ChIP-Seq analysis in Jurkat T cells with ChIP-PCR validation in primary CD4+ T cells; B) determine how Pbx1-d affects chromatin structure, TGF and ATRA responses in vitro; and C) validate that Pbx1-d affects CD4 T cell effector function in vivo. Aim 2. To determine the mechanisms by which Esrrg regulates CD4+ T cells and contributes to lupus. We will A) assess the effect of Esrrg deficiency on T cell functions and lupus; B) characterize the mechanisms by which Esrrg regulates T cell metabolism by combining techniques to analyze metabolism parameters, mitochondrial function, and gene expression; and C) test the hypothesis that Esrrg regulates T cell functions through metabolism. . Aim 3. To determine how PBX1-d or ESRRG expression affect CD4+ T cell functions in SLE patients. A) We will determine the mechanisms by which PBX1-d expression affect CD4+ T cells by comparing immune gene expression in sorted nave, memory, CD25+ Tregs and CD25- Tregs obtained from SLE patients and healthy controls (HC) partitioned on Pbx1-d expression. Functional validation will be conducted T cells in basal conditions as well as after activation or polarization with TGF and ATRA; B). We will compare metabolic substrate utilization between SLE and HCs CD4+ T cells and determine whether it correlate with Esrrg expression level, effector functions and gene expression. We expect these results to delineate two novel regulatory pathways of T cell effector functions, to provide novel biomarkers for autoreactive T cells as well as identify lupus therapeutic targets.